>I did full reset including removing backup battery. Did not >help.
The backup battery? If you're using a grip, is it a Nikon or a third-party grip? If it's a third-party grip, there could be an electronic problem that is causing random slowdowns. As well, with any grip, an accidental half-press/nudge of the shutter button on the grip will activate the buffer and slowdown whatever card write is taking place. I'm just guessing at possible causes, because I've never encountered the problem you described.